Piezoelectric end capped cylinder assembly for use to the radial-mode resonance frequency

ABSTRACT

A cylindrical end capped piezoelectric transducer element provided with a concentrically mounted interior post cemented to the end caps. The structure prevents resonance of the end caps to thus provide uniform, relatively flat response to the radial mode resonance frequency.

Sttes tent 91 Tims et al.

[ Aug. 6, B974 PIEZOELECTRIC END CAPPED CYLINDER ASSEMBLY FOR USE TO THE RADIAL-MODE RESONANCE FREQUENCY [75] Inventors: Allan C. Tims; Theodore A.

Henriquez, both of Orlando, Fla.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

22 Filed: June2l, 1972 21 Appl.No.:265,042

[52] US. Cl. 179/110 A, 340/10 [51] Int. Cl H04r 17/00 [58] Field of Search 189/110 A, 110 F; 310/82,

[56] References Cited UNITED STATES PATENTS 2,945,208 7/1960 Samsel 340/l0 3,437,171 4/1969 Davis et al. 340/10 Primary Examiner-Kathleen H. Claffy Assistant ExaminerThomas L. Kundert Attorney, Agent, or FirmR. S. Sciascia; Arthur L. Branning [5 7] ABSTRACT A cylindrical end capped piezoelectric transducer element provided with a concentrically mounted interior post cemented to the end caps. The structure prevents resonance of the end caps to thus provide uniform, relatively flat response to the radial mode resonance frequency.

3 Claims, 2 Drawing Figures PAIEN {En AUG 6 I914 DECIBELS RE I VOLT PER MICROPASCAL FREQUENCY IN kHz PIEZOELECTRIC END CAPPED CYLINDER ASSEMBLY FOR USE TO THE RADIAL-MODE RESONANCE FREQUENCY BACKGROUND OF THE INVENTION The resonance-free frequency band for a cylindrical piezoelectric ceramic element is a function of the largest dimension of the cylinder, which is usually the circumference; because the circumference is a function of the radius, the mean radius can be said to determine the first resonance called the radial-mode resonance. Although a smooth, relatively flat response should result at all frequencies up to that of the radialmode resonance, present capped cylinders operate smoothly only to frequencies between 40% and 60% of the radial-mode resonance frequency. The useful response terminates at a lower frequency, because the end caps of the cylinder break into flexure resonance causing a drop of several dB in sensitivity. When several of the transducers are formed into a closely spaced line array the resulting change in dB becomes greatly pronounced.

In tests a inch diameter cylinder without the posts showed end cap break up at 25 kHz while the radialmode resonance should occur at 50 kHz.

With a post the response of the transducer is relatively smooth and broadened up to the radial-mode resonance frequency.

Thus it is seen that with the use of the post the element is usable with a much broader bandwidth up to the radial-mode resonance where prior art devices without the posts were usable only over about 60 percent of the possible usable range.

SUMMARY OF THE INVENTION A cylindrical end capped piezoelectric transducer element provided with an interior post cemented to the end caps. The structure prevents flexure resonance of the end caps to thus provide uniform, relatively flat responses to the radial-mode resonance frequency.

OBJECTS OF THE INVENTION An object of the invention is to provide an improved end capped transducer.

Another object is the provision of an extended frequency range for end capped transducers.

Yet another object is to provide a transducer which can sustain greatly increased operating pressures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a section view of the cylindrical transducer preferred embodiment.

FIG. 2 shows a comparison of the sensitivity for a capped two element line with and without posts.

PREFERRED EMBODIMENT OF THE INVENTION Now referring to FIG. 1 shich shows a sectional side view of the transducer assembly, the transducer consists of a cylindrical piezoelectric ceramic element 1,

having aluminum oxide or beryllium oxide end caps 2 bonded by an epoxy adhesive 3 to the ends of said element 1. A glass post 4 of the exact length of the cylinder is concentrically mounted inside the cylindrical element 1 and is also epoxy bonded to the end caps 2. The crosssectional area of the post is approximately 40 percent of that of the end cap.

The post allows the response of the transducer to be useful over a broad range of frequency up to the radialmode resonance while prior art transducers without such a structure were only usable to about 60 percent of the radial-mode resonance frequency.

The improved transducer is of great use in line arrays where previous transducers appeared to have a severe drop in sensitivity due to end flexure resonance and loss change in directivity characteristics.

Thus using the improved transducer line arrays with broad bandwidth relatively smooth response and predictable directivity are possible up to the radial-mode resonance frequency while retaining the well-known stability with temperature and pressure.

The high-pressure capability of the transducer is improved as a result of the mechanical support that the post gives to the end caps. Further it should be noted the structure is mechanically sound with support passing through the end caps to supply support.

FIG. 2 shows a comparison of the sensitivity of a two element line with and without the post. The sensitivity without the post curve 5 drops severely at about 30 kHz while the sensitivity with the post curve 6 remains relatively stable through the range of the element. As the severity of the sensitivity loss increases with the increase in the length of the line (apparently due to sympathetic resonance of end caps of other elements which are in a close proximity) the use of the post is of great value in maintaining a broad frequency band of response.

Of course many other materials can be used for the post and the end caps but optimum bandwidth has been achieved with the materials specified.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. An electroacoustic transducer which comprises:

a hollow cylindrical piezoelectric crystal;

a first end cap bonded to a first end of said crystal;

a second end cap bonded to a second end of said crystal; and,

a cylindrical post concentrically disposed within said crystal and bonded at each end to the internal side of said end caps.

2. The transducer of claim 1 wherein said end caps are composed of beryllium oxide.

3. The transducer of claim 1 wherein said end caps are composed of aluminum oxide. 

1. An electroacoustic transducer which comprises: a hollow cylindrical piezoelectric crystal; a first end cap bonded to a first end of said crystal; a second end cap bonded to a second end of said crystal; and, a cylindrical post concentrically disposed within said crystal and bonded at each end to the internal side of said end caps.
 2. The transducer of claim 1 wherein said end caps are composed of beryllium oxide.
 3. The transducer of claim 1 wherein said end caps are composed of aluminum oxide. 